1. Field of the Invention
This invention relates to an apparatus for reading out a radiation image stored on a stimulable phosphor sheet. This invention particularly relates to a radiation image readout apparatus wherein light emitted by the stimulable phosphor sheet upon stimulation thereof in proportion to the radiation energy stored is detected accurately.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy.
FIG. 7 is a schematic view showing an example of a radiation image read-out apparatus employed in the aforesaid radiation image recording and reproducing system.
In the apparatus of FIG. 7, a laser beam 101a of a predetermined intensity is emitted as stimulating rays from a laser beam source 101 to a galvanometer mirror 102. The laser beam 101a is deflected by the galvanometer mirror 102 to form a laser beam 101b impinging upon a stimulable phosphor sheet 103 positioned below the galvanometer mirror 102 so that the sheet 103 is scanned by the laser beam 101b in the main scanning direction, i.e. in the width direction of the sheet 103 as indicated by the arrow A. While the laser beam 1b impinges upon the stimulable phosphor sheet 103, the sheet 103 is conveyed in the sub-scanning direction as indicated by the arrow B, for example, by an endless belt device 109. Therefore, scanning in the main scanning direction is repeated at an angle approximately normal to the sub-scanning direction, and the whole surface of the stimulable phosphor sheet 103 is two-dimensionally scanned by the laser beam 101b. As the stimulable phosphor sheet 103 is scanned by the laser beam 101b, the portion of the sheet 103 exposed to the laser beam 101b emits light having an intensity proportional to the stored radiation energy. The light emitted by the stimulable phosphor sheet 103 enters a transparent light guide member 104 from its light input face 104a positioned close to the sheet 3 in parallel to the main scanning line. The light guide member 104 has a flat-shaped front end portion 104b positioned close to the stimulable phosphor sheet 103 and is shaped gradually into a cylindrical shape towards the rear end side to form an approximately cylindrical rear end portion 104c which is closely contacted with a photomultiplier 105. The light emitted by the stimulable phosphor sheet 103 upon stimulation thereof and entering the light guide member 104 from its light input face 104a is guided inside of the light guide member 104 up to the rear end portion 104c, and received 15 by the photomultiplier 105. Thus the light emitted by the stimulable phosphor sheet 103 in proportion to the radiation energy stored therein is detected and converted into an electric image signal by the photomultiplier 105. The electric image signal thus obtained is sent to an image processing circuit 106 and processed therein. The electric image signal thus processed is then reproduced into a visible image and displayed, for example, on a CRT 107, or stored in a magnetic tape 108, or directly reproduced as a hard copy on a photographic material or the like.
In this manner, the radiation image stored on the stimulable phosphor sheet 103 is read out. However, since the light input face 104a of the light guide member 104 extends approximately over the entire width of the stimulable phosphor sheet 103 in parallel to the main scanning line thereon, all light emitted by the portions of the stimulable phosphor sheet 103 covered by the light input face 104a enters the light guide member 104 from the light input face 104a and is detected by the photomultiplier 105. That is, not only the light emitted by the portion of the stimulable phosphor sheet 103 upon which the laser beam 101b impinges at any given instant, in proportion to the radiation energy stored in that portion, but also the other light emitted as described below by the portions of the sheet 103 covered by the light input face 104a enters the light guide member 104 and is detected by the photomultiplier 105. The light other than the light emitted by the portion of the stimulable phosphor sheet 103 upon which the laser beam 101b impinges at any given instant in proportion to the radiation energy stored in that portion includes after-glows emitted by the stimulable phosphor sheet 103. The after-glows are divided into an instantaneous light emission after-glow and a stimulated light emission afterglow.
By "instantaneous light emission after-glow" is meant the after-glow of light instantaneously emitted by a stimulable phosphor sheet when the sheet is exposed to a radiation to have a radiation image stored in the sheet, the after-glow continuing to be emitted by the sheet while the light intensity decays after the exposure of the sheet to the radiation is ceased. The characteristics of the instantaneous light emission after-glow are generally as shown in FIG. 8, though they will differ depending on the type of the stimulable phosphor constituting the stimulable phosphor sheet. In the graph of FIG. 8, the ordinate represents the intensity of light emission and the abscissa represents time (t). As shown in FIG. 8, when the exposure of a stimulable phosphor sheet to a radiation is ceased after the sheet is exposed to the radiation for a period of .DELTA.t2 from a time t1 to a time t2, the intensity of light instantaneously emitted from the sheet at a light emission intensity A does not immediately decrease to zero, but instead an instantaneous light emission after-glow continues while the intensity thereof decreases along an exponential function curve the time constant of which increases gradually.
For example, decay of the light emission intensity of the instantaneous light emission after-glow is such that a light emission intensity B of the instantaneous light emission after-glow at a time t3 approximately 180 seconds after the exposure of a stimulable phosphor sheet to a radiation is ceased (i.e. t3-t2=180 seconds) is approximately 10.sup.-4 times the intensity of light emitted by the sheet when the sheet is exposed to stimulating rays.
Accordingly, in the case where a predetermined time elapses from when a stimulable phosphor sheet is exposed to a radiation passing through an object to have a radiation image stored in the sheet to when read out of the radiation image stored is conducted, the intensity of the instantaneous light emission after-glow decreases sufficiently and the effect of the after-glow becomes negligible in the read-out step. However, when read-out of the radiation image is conducted immediately after the radiation image is stored in the stimulable phosphor sheet, for example when a radiation image recording and read-out apparatus as described in U.S. patent application No. 600,689 wherein an image recording section and an image read-out section are installed integrally to record and read out many radiation images continuously and quickly is employed, the light emission intensity of the instantaneous light emission after-glow does not decay sufficiently before image read-out is conducted. As a result, the instantaneous light emission after-glow is detected together with the light emitted by the stimulable phosphor sheet in proportion to the radiation energy stored when the sheet is exposed to stimulating rays, and the effect of the instantaneous light emission after-glow on the electric image signals obtained thereby becomes large.
Further, the light emission by the stimulable phosphor sheet upon stimulation thereof by stimulating rays arises from a portion having a very small area upon which the stimulating rays impinge, whereas the instantaneous light emission after-glow is emitted by the whole surface of the stimulable phosphor sheet exposed to a radiation. Therefore, as the stimulable phosphor sheet 103 is scanned point by point by the laser beam 101b as shown in FIG. 7, the light emitted by a portion of the sheet 103 upon which the laser beam 101b impinges momentarily in proportion to the radiation energy stored in that portion and the instantaneous light emission after-glow emitted by all of the portions covered by the light input face 104a of the light guide member 104 simultaneously enter the light guide member 104 from the light input face 104a and are guided to the photomultiplier 105. In this case, since the area of the portions covered by the light input face 104a of the light guide member 104 is markedly larger than the area of the portion of the stimulable phosphor sheet 103 upon which the laser beam 101b impinges momentarily, the amount of the instantaneous light emission after-glow guided to the photomultiplier 105 becomes not negligible even though a predetermined time elapses after the exposure of the stimulable phosphor sheet 103 to a radiation is ceased and the intensity of the instantaneous light emission after-glow becomes negligibly low as compared with the intensity of the light emitted by the sheet 103 upon stimulation thereof.
By "stimulated light emission after-glow" is meant the after-glow of light emitted by a stimulable phosphor sheet carrying a radiation image stored therein when the sheet is exposed to stimulating rays (e.g. a laser beam) for reading out the radiation image, the after-glow continuing to be emitted by the sheet while the light intensity decays after the exposure of the sheet to the stimulating rays is ceased. The characteristics of the stimulated light emission afterglow are generally as shown in FIG. 9, though they will differ depending on the type of the stimulable phosphor constituting the stimulable phosphor sheet. In the graph of FIG. 9, the ordinate represents the intensity of light emission and the abscissa represents the time (t). As shown in FIG. 9, when the exposure of a stimulable phosphor sheet to stimulating rays is ceased after the sheet is exposed to the stimulating rays for a period of t5 from a time t4 to a time t5, the intensity of light emitted by the sheet upon stimulation thereof at a light emission intensity C does not immediately decrease to zero, but instead a stimulated light emission after-glow continues while the intensity thereof decreases along an exponential function curve with the time constant thereof increasing gradually. (That is, the light intensity decreases rapidly at the beginning and thereafter the rate of decrease in the light intensity becomes gradually lower.)
For example, decay of the light emission intensity of the stimulated light emission after-glow is such that the initial time constant is approximately one microsecond, i.e. the time t6-t5 required for the light emission intensity to become 1/e (D/C =1/e) is approximately one microsecond. In general, since the speed of scanning (in the main scanning direction ) of a stimulable phosphor sheet by stimulating rays by use of a galvanometer mirror is approximately 50 Hz, it takes approximately 20,000 microseconds for scanning one time. Accordingly, the intensity of the stimulated light emission after-glow decaying along an exponential function curve with the initial time constant of one microsecond becomes very low as compared with the intensity of the light emitted by the stimulable phosphor sheet upon stimulation thereof when the sheet is exposed to the stimulating rays. Thus the intensity of the stimulated light emission after-glow at each point of the stimulable phosphor sheet becomes almost negligible.
However, the light emission by the stimulable phosphor sheet upon stimulation thereof when the sheet is exposed to stimulation rays arises from a portion having a very small area upon which the stimulating rays impinge, whereas the stimulated light emission after-glow is emitted by the whole surface of the stimulable phosphor sheet scanned by the stimulating rays. Therefore, as the stimulable phosphor sheet 103 is scanned point by point by the laser beam 101b as shown in FIG. 7, the light emitted by a portion of the sheet 103 upon which the laser beam 101b impinges momentarily in proportion to the radiation energy stored in that portion and the stimulated light emission after-glow emitted by all of the portions covered by the light input face 104a of the light guide member 104 simultaneously enter the light guide member 104 from the light input face 104a and are guided to the photomultiplier 105. In this case, since the area of the portions covered by the light input face 104a of the light guide member 104 is markedly larger than the area of the portion of the stimulable phosphor sheet 103 which is momentarily exposed to the laser beam 101b and which emits light upon stimulation by the laser beam 101b, the amount of the stimulated light emission after-glow guided to the photomultiplier 105 becomes not negligible even though the intensity of the stimulated light emission after-glow becomes negligibly low as compared with the intensity of the light emitted by the sheet 103 upon stimulation thereof.
The after-glows detected together with the light emitted by the stimulable phosphor sheet upon stimulation thereof by stimulating rays as described above constitutes a noise component in the electric image signals obtained by the read-out of a radiation image and make it difficult to accurately read out the radiation image.
The instantaneous light emission after-glow presents a problem particularly when image read-out is carried out immediately after a stimulable phosphor sheet is exposed to a radiation to have the radiation image stored therein. On the other hand, the stimulated light emission after-glow presents a problem particularly when the scanning speed of stimulating rays on the stimulable phosphor sheet carrying the radiation image stored therein is increased.
The effects of the after-glows on the amount of light detected by image read-out will hereinbelow be described in more detail with reference to FIGS. 10A and 10B. FIG. 10A shows a stimulable phosphor sheet 103a carrying a radiation image of a human head stored therein. FIG. 10B shows a graph wherein the abscissa represents the scanning point along the line a on the stimulable phosphor sheet 103a of FIG. 10A and the ordinate represents the amount of light transmitted to a photomultiplier via a light guide member when the stimulable phosphor sheet 103a is scanned by stimulating rays (laser beam) along the line a. In FIG. 10B, the broken line l1 designates the amount of light actually transmitted to the photomultiplier, and the solid line l2 designates the amount of light emitted by the stimulable phosphor sheet 103a upon stimulation thereof when the sheet 103a is exposed to the stimulating rays. The chain line l3 designates the amount of after-glows (i.e. the sum of the instantaneous light emission after-glow and the stimulated light emission after-glow). That is, the sum of the amount l3 of the after-glows and the amount l2 of the light emitted by the stimulable phosphor sheet 103a upon stimulation thereof when the sheet 103a is exposed to the stimulating rays is equal to the light amount l1 transmitted to the photomultiplier. The light amount l1 is converted to an electric image signal by the photomultiplier and then logarithmically converted to reproduce a visible image by use of the logarithmically converted signal. In this case, the signal level obtained when the light amount l1 transmitted to the photomultiplier is converted to an electric image signal and then logarithmically converted is different from the signal level obtained when only the amount l2 of light emitted by the stimulable phosphor sheet 103a upon stimulation thereof by the stimulating rays is converted to an electric image signal and then logarithmically converted. Therefore, when a visible image is reproduced by use of the image signal obtained by converting the light amount l1 transmitted to the photomultiplier, the visible image thus reproduced becomes different from the correct image. That is, the visible image reproduced becomes incorrect or unsharp, and a very real problem arises with regard to the image quality, particularly diagnostic efficiency and accuracy.
Besides the after-glow problems described above, there often arises the problem that a part of the laser beam 101b is reflected by the surface of the stimulable phosphor sheet 103, and the reflected light is further reflected by the light input face 104a of the light guide member 104 to a non-scanned portion of the sheet 103 outside of the scanned portion thereof, thereby stimulating the stimulable phosphor at the non-scanned portion to emit light. When the light emitted by the non-scanned portion of the stimulable phosphor sheet 103 outside of the scanned portion thereof is detected by the photomultiplier, the light constitutes a noise component in the electric image signal obtained thereby, and the sharpness of the image reproduced by use of the electric image signal is deteriorated.
In order to eliminate the aforesaid problems, the applicant proposed in U.S. patent application No. 642,868 a radiation image read-out apparatus provided with a means for preventing the instantaneous light emission after-glow, the stimulated light emission after-glow, and the light emitted by a non-scanned portion of the stimulable phosphor sheet outside of the scanned portion thereof upon stimulation of the non-scanned portion by the reflected stimulating rays from entering the light guide member.
In the radiation image read-out apparatus, an aperture member provided with an aperture for allowing the stimulating rays to impinge upon the stimulable phosphor sheet for scanning it in the main scanning direction and for allowing the light emitted by the sheet upon stimulation thereof by the stimulating rays to enter the light input face of the light guide member, and light shielding sections positioned adjacent the aperture on the front side and the rear side thereof in the sub-scanning direction is positioned between the surface of the sheet and the light input face of the light guide member. However, since the light guide member is positioned close to the sheet in order to increase the efficiency with which the light emitted by the sheet is guided, it is not always possible for spatial reasons to position the aperture member between the sheet and the light input face of the light guide member. Further, in order to minimize the light guiding area on the sheet without cutting off the light emitted by the sheet, which should be detected, by the aperture member, the aperture of the aperture member must be made as small as the light guiding area and the aperture member must be positioned so that it almost contacts the sheet. However, it is not always possible to position the aperture member in this manner and to fabricate the aperture member having a very narrow aperture.